In conventional photolithographic processing of integrated circuits, a wafer having photosensitive chemicals deposited thereon is exposed through a mask or reticle. The mask or reticle has a pattern of clear and opaque chrome features that will selectively expose areas of the wafer much like a photolithographic negative. The exposed wafer is then further processed in order to create the circuit components of an integrated circuit.
While conventional chrome-on-glass photolithographic masks work well for many technologies, it is difficult to print the smallest features of the latest circuit designs with chrome patterns. One technique used to print the smallest features of a layout is by using phase shifters. A phase shifter operates to shift the phase of the illumination light by some amount, typically 180 degrees, with respect to light that passes through an adjacent, non-phase shifting area of the mask. The phase shifted and non-phase shifted light destructively interfere on the surface of the wafer to create desired patterns on the wafer.
As illustrated in FIG. 1A, it is well known to those skilled in the art that the image of a single chromeless 0°-180° phase transition 2 on a mask forms an image with a single interference fringe that forms a single very thin dark line. It is also known that two 0°-180° phase transitions 4 far apart print as two distinct thin lines, but that under certain conditions, depending on the illumination wavelength and angle as well as the imaging system numerical aperture, these two features will merge into a single dark line when the two transitions are close enough together such as 6. Larger numbers of phase transitions 8 will merge into a single large dark area, as long as the spacing between any two transitions is less than the separation at which these phase transitions merge for that particular optical system. Masks using these techniques, in which regions traditionally formed with opaque mask structures have been replaced with transparent twin phase edges or larger phase gratings are generally called “chromeless phase masks”.
While phase shifting technology is relatively well known in the art, it is often difficult to create the masks that use phase shifters for the smallest components as well as other exposure techniques for larger features. Therefore, there is a need for a mechanism for converting a layout into mask features that can print both small and larger features and can be easily manufactured.